1. Field of the Invention
This invention relates to a method of and apparatus for treating granular materials which require to be heated to drive off volatile constituents, and which require to be cooled and, more particularly, but not exclusively, foundry sand containing an organic binder.
2. Description of the Prior Art
Because of rising costs of transport for clean sand deliveries and environmental difficulties associated with dumping, it is becoming increasingly desirable to treat used foundry sand, to convert the used sand, i.e. sand grains covered with spent resin and resin dust, to clean sand for re-use.
A percentage of such used sand can be treated for re-use using only mechanical attrition. However this process does not remove all, and in some cases hardly any, of the resin binder and the presence of the residual spent binder is a problem with some binder systems, particularly the furane resin-peroxide-sulphur dioxide gas hardening system.
The only good reclamation system for such organic binder systems is one in which the organic component is burned off. Conventionally however such systems are gas heated, and because natural gas flames are difficult to sustain at temperatures below 800.degree. C., most existing thermal reclamation systems work in the temperature range 800.degree.-1000.degree. C. Such systems include fluid beds and rotary kilns. These existing processes have high capital investment costs and high energy consumption, in the region of 300 kwh/tonne of sand. Much of this large energy input is required by the cooling systems designed to reduce the temperature of the sand from red hot to about 35.degree. C., at which it can be reused.
U.S. Pat. No. 2,478,461 discloses a method and apparatus in which sand is heated in a furnace to a temperature of from 650.degree. C. to 816.degree. C. and cooled by mixing with cool sand which has been previously so heated and then cooled.
One fluid fired fluid bed reclaimer is known to work at a lower temperature of 500.degree. C., and reclaims satisfactorily, producing a weight loss on ignition of the reclaimed sand below 0.01 wt% and a best performance of 100 kwh per ton. However, it is known to suffer from flame failure and other stability and control problems. It is not easy to run and its floor space requirements are high.
U.S. Pat. No. 3,685,165 discloses a method and apparatus in which sand to be reclaimed is passed via a plurality of pre-heating chambers to an electrically heated chamber where the sand is at a temperature of about 650.degree. C. and then discharged via a series of cooling chambers, heat from the sand in the cooling chambers serves to heat the sand in the pre-heating chambers.
U.S. Pat. No. 3,480,265 discloses a method and apparatus in which previously reclaimed sand is heated to 593.degree. C. in a fluidised bed and then used sand is added to the bed so as to be heated to 593.degree. C. to burn off carbonised resin material. The hot thus reclaimed sand is then discharged. The bed can be electrically or fluid fuel heated.
These previous methods are all methods of treating granular material which comprise a step of maintaining a first mass of said granular material at a treatment temperature for a time sufficient to achieve desired temperature of the material wherein the treatment temperature is at least 593.degree. C. and suffer from the disadvantages discussed above.